Selective removal of some heavy metals from Lanthanide solution by graphene oxide functionalized with sodium citrate

Lanthanides are widely used in several advanced technologies, and the presence of heavy metal ions as traces reduce their efficiencies. Hence, adsorption of some heavy metals from Lanthanides aqueous solution using previously prepared graphene oxide-citrate (GO-C) composite was reported. In this regard, the GO-C was applied to remove various heavy metal ions (Fe, Ni, Mn) through the batch technique. The GO-C after the adsorption process was characterized by various advanced techniques. The results obtained from the experimental investigations revealed that the GO-C showed a rapid equilibrium adsorption time (1.0 min) for all the studied heavy metal ions. Moreover, the adsorption isotherm data for Fe3+, Mn2+, and Ni2+ was fit by the Langmuir isotherm model with excellent adsorption capacity for Fe3+ (535.0 mg/g), Mn2+ (223.22 mg/g), and Ni2+ (174.65 mg/g). Furthermore, the GO-C can be reused over five times to enhance the removal efficiency. Interestingly, the GO-C adsorbent achieved removal performance reached 95.0% for the Fe3+ and ≥ 35.0% for Ni, Mn, Co, and Cu compared to < 1% for lanthanides metal ions.

Preparation and characterization of GO-C composite. The composite was prepared according to our previous work 37 . The properties of citrated modified graphene oxide after adsorption were studied using SEM, EDX, FT-IR and Raman Spectroscopy. Surface morphology was identified using a JEOL SEM Model, JSM-6510A, Japan. The IR investigations were performed using an FTIR spectrometer, PerkinElmer, model 1600, USA. The elemental composition of Ni(II), Mn(II), and Fe(III) sorption on citrate-modified graphene oxide was detected by an Oxford energy-dispersive X-ray (EDX) spectrometer (Oxford Link ISIS, Japan). A Shimadzu UV-Visible double beam spectrophotometer (model UV-160A, Japan), was used for all spectrophotometric measurements.
Batch sorption procedure. An iron (III) (1.0 g/L) stock solution was prepared by dissolving a known amount of iron chloride in minimum concentrated hydrochloric acid and evaporating it to near dryness, then formed to the mark with double-distilled water. Manganese and nickel solutions and standards (1.0 g/L) were prepared by dissolving a certain amount of manganese sulfate monohydrate and nickel sulfate hexahydrate in distilled water. The required concentrations of test solutions were prepared by appropriate dilution of the stock solutions.
The heavy metal ion concentrations of Ni 2+ and Mn 2+ were separately determined using the 4-(pyridyl-2-azo) resorcinol (PAR) method 38 . The concentration of iron was also determined using the thiocyanate method 38 . Batch sorption experiments were carried out by shaking 2.4 g (~ 0.2 mL) of the prepared citrated graphene oxide with 5.0 mL of known concentration of each metal ion aqueous solution in a thermostatic shaker bath at a constant temperature for a predetermined period. Metal ions adsorption was calculated as the difference between initial metal ions concentration in solution and its concentration after shaking time (t). The number of ions retained in the solid phase at equilibrium (q e ) in (mg/g) was calculated using the following equation: where C o and C e are the initial and equilibrium concentrations in (mg/L) of ions solution, respectively, V is the volume of solution in (L), and m is the weight of the adsorbent in (g).

Results and discussion
Preliminary investigations showed that citrate-modified graphene oxide (GO-C) can eliminate heavy metals from aqueous solution due to active functional groups (carboxylic group) of the citrate. Therefore, sorption investigations of the relevant metal ions were performed by the (GO-C) from an aqueous solution.
Characterization of modified GO-C-M. Different techniques, such as SEM, FTIR, Raman, and EDX analysis, were used to characterize the citrate-modified graphene oxide-heavy metal ions (GO-C-M) complex to assess the adsorption process.
SEM analysis. The morphology of the GO-C composite was previously investigated by SEM and TEM techniques 37 . The SEM images presented that, the GO-C composite appears as a layered structure of GO loaded with the modifier. Further, the TEM images indicated that the GO-C is composed of the fully separated layered structure of GO decorated with dark spots of the modifier 37 .
The SEM images of the adsorbent-metal ion complexes (GO-C-M, M = Fe, Ni, or Mn) are presented in Fig. 1. The GO sheets were modified with sodium citrate, which has three full ionized carboxylate groups. These groups exhibited high interaction affinity with the metal ions. Therefore, introducing the GO-C in an aqueous solution of heavy metal ions tends to form a strong complex with this metal ion. This behavior changed the flat morphology of the GO-C to shrinkage structure of GO-C-M, as seen in Fig. 1. Raman spectra. Raman spectroscopy is widely used to explore the structure change for GO to new functionalized graphene oxide. The Raman spectrum of GO 37 shows two bands of D-band at 1352 cm −1 and the G-band at 1598 cm −1 . It is well known that the G-band is related to the vibration of the sp 2 carbon atoms in the graphitic 2D hexagonal lattice. On the other hand, the D-band reflects the disorder and local defects. This technique was used in terms of analysis of the location, intensities, and border of the D-band and G-band, as seen in Fig. 2b. The locations of the D and G-bands and the values of I D /I G ratios and FWHMs are summarized in Table 1.
In the case of adsorption of Fe 3+ on the GO-C composite, the I D /I G ratio of the GO-C-Fe was 1.136, which is less than the I D /I G ratio for the GO-C composite (1.27). While in the case of Ni 2+ and Mn 2+ , the I D /I G ratio for GO-C-Ni and GO-C-Mn is 1.550 and 1.780, respectively. This result suggests that the type and the oxidation state of the metal ion affected the defect states (sp 2 /sp 3 plane) of the GO-C composite.
EDX analysis. The importance of EDS analysis highlights the elemental composition of the fabricated material. Graphene oxide is a carbonaceous material mainly composed of C and O-atoms. Herein, we modified the GO with tri-sodium citrate and used tetraethylorthosilicate (TEOS) as a binder. Therefore, the elemental analysis of GO-C shows the presence of Na and Si atoms in the resulting EDS analysis 37 , see Fig. 3. When GO-C was used in treating aqueous solutions of Fe 3+ , Ni 2+ , and Mn 2+ , the M ions were expected to bind with the composite at the carboxylate groups (-COO − Na + ) to form (-COO − M + ) and release the Na + . Hence, in the analysis of the GO-C-M, the M-ions will appear in the results instead of Na + , as presented in Fig. 3.

Batch investigations.
Preliminary batch investigations were carried out to assess the time required for the adsorption equilibrium, pH, V/m ratio for statistically acceptable adsorption values, as well as the effect of the initial metal ions concentrations and temperature.
Effect of contact time. In practical application, the adsorption contact time is a very important factor. The influence of the contact time (1.0-30.0 min) on the uptake percent of Fe 3+ (100.0 mg/L, pH = 2.0), Ni 2+ (50.0 mg/L, pH = 5.0), and Mn 2+ (100.0 mg/L, pH = 5.0) and V/m ratio was kept at 2.1 L/g for all metal ions using the GO-C was plotted in Fig. 4a. The adsorption behavior recorded a high removal efficiency in the early stages (1.0 min) and remained nearly constant with the further increase in the contact time. This fast adsorption may be related to the flat structure of the composite 37 , which makes a large number of the carboxylated functional (-COO − Na + ) exposed to the adsorbed metal ions. Moreover, the carboxylated groups tend to form a complex with the M-ion.
Effect of aqueous solution pH. The initial pH of the solution is a significant parameter that stimulates the adsorption process. It is affected not only by the adsorbent surface charge but also by the degree of the adsorbate ionization. Here, the effect of pH aqueous solution on the uptake percent for the studied metal ions in the range (0.5-2.5 for Fe 3+ , 2.0-8.0 for Ni 2+ , 2.0-7.0 for Mn 2+ , 1.0-5.0 for La 3+ , and 1.0-5.0 for Nd 3+ ) was investigated and As previously proposed 37 , the used GO-C composite contains three sodium carboxylate groups in the solid phase. In an aqueous solution at low pH, the sodium ions will exchange with the H + in the solution. This will form mono and diprotonated citrate on the surface of the composite. Based on the Medusa program (www. kemi. kth. se/ medusa), the speciation diagram for citric acid as a function of pH is given in Fig. S1. From this Mn 2+ ) of the tested metal ions using GO-C composite is given in Fig. 4c. It is observed that the amount of the adsorbed metal ions (Fe 3+ , Ni 2+ , and Mn 2+ ) increases with the increase of the initial concentration of the metal ions in the tested range. This observation can be indicated by increasing the initial metal ion concentration, which leads to an increase in the concentration gradient, which is performed as a driving force to reduce the resistance to mass transfer of the metal ion from the bulk of solution to the adsorbent surface. Then, the affinity of the binding sites for interaction with the metal ions increases, and thus, the adsorption capacity is enhanced.
Effect of V/m ratio. In order to evaluate the optimum GO-C weight, which donated the highly acceptable adsorption values, the induced V/m ratio (L/g) in the range (4.2-0.7 for Fe 3+ , and 2.1-0.83 for both Ni 2+ and Mn 2+ ) on the uptake percent of the studied metal ions from aqueous solution was investigated (Fig. 4d). The uptake percent was increased as the V/m ratio decreased for the three metal ions. The optimum V/m ratio was chosen at 0.83 L/g for Fe 3+ and 1.04 L/g for both Ni 2+ and Mn 2+ . Adsorption kinetic model, sorption isotherm model, and thermodynamics. The adsorption kinetics were investigated to assess the rate-controlling step, mass transfer, and chemical reaction process. As presented in the effect of mixing time section, the adsorption equilibrium reached high rapidly (1.0 min). Therefore, the adsorption kinetics were investigated employing the pseudo-second-order (see Supplementary Materials). The linear relation between the t and t/q t was plotted in Fig. 5a, and different parameters were calculated and listed in Table 2. It was observed that the correlation coefficient is R 2 ≥ 0.995. Moreover, the calculated adsorption capacity was closer to the experimental adsorption capacities values. These findings suggested that the adsorption kinetics are excellently fitted with pseudo-second-order, which indicates that chemical adsorption is more predominant. This result can illustrate that the mechanism of Fe 3+ , Ni 2+ , and Mn 2+ onto GO-C is controlled by the exchange mechanism. Adsorption isotherm is important to design the adsorption systems. Moreover, it explains the relationship between the amount of adsorbate uptake from the aqueous phase using a unit mass of the adsorbent at a constant temperature. Equilibrium isotherm modeling was performed using Langmuir, Freundlich, Dubinin-Radushkevich, Temkin, and Flory-Huggins isotherms (see Supplementary Materials). Furthermore, the linear isotherm modeling plots are shown in Fig. 5b-f, respectively. The correlation coefficient and adsorption isotherm parameters of different models were evaluated and summarized in Table 3.
We noted that the values of R 2 for all the studied metal ions (Fe 3+ , Ni 2+ , and Mn 2+ ) related to the Langmuir model were (0.999) closer to the unit. Moreover, the maximum adsorption capacities, mg/g, were 531.91 (Fe 3+ ), 171.23 (Ni 2+ ), and 223.22 (Mn 2+ ). Moreover, the sorption Langmuir energy (b) values for the metals studied were greater than zero, explaining that Langmuir is the appropriate model. The R L values were < 1.0, and > 0 indicates high favorable sorption of Fe 3+ , Ni 2+ , and Mn 2+ on GO-C for all studied concentrations. Moreover, according to the error function data in Table S1, it is clear that Langmuir is the best model to describe the adsorption data. The Langmuir isotherm assumes that the solid surface has a finite number of identical sites that are energetically uniform. According to the Langmuir model, there is no interaction between adsorbed species, which means that the adsorbed amount did not influence the adsorption rate. A monolayer was formed when the equilibrium was attained.
To further optimize the thermodynamic parameters (see Supplementary Materials) of the adsorption process, Gibbs free energy (ΔG°), Enthalpy (ΔH°), and Entropy (ΔS°) were detected related to Fig. 5g, and the measured  Table 4. The increase in negative values of the ΔG° with a further increase in the temperature reveals that the metal ions interact spontaneously with the GO-Cit surface. On the other hand, the values of ΔH° and ΔS° are tabulated in Table 4. This table shows that the positive values of ΔH° refer to the endothermic type of the sorption process, while the positive values of (ΔS°) show an increase in the randomness of the system. Moreover, the affinity of the GO-C towards the metal ion increase with temperature rises.
Regeneration and reusability. The ability to release the adsorbed metal ion from the binding site on the adsorbent is a significant factor in evaluating the economic efficiency and applicability of the adsorbent used. Thus, the regeneration of the GO-C composite was studied. Herein, the adsorbent firstly adsorbed the M-ion. Secondly, 10.0% of HCl was selected to liberate the M-ion from the adsorbent binding site and washed with distilled water. Finally, the GO-C composites were activated with 10.0% NaOH and washed with distilled water. The regenerated GO-C was reused to adsorb the metal ion again, as illustrated in Fig. 6a. It was obviously noted that the regenerated GO-C composite shows a little higher removal efficiency than that of pristine composite, which increases the composite evaluability and applicability.

Selective adsorption of different heavy metal ions from lanthanide aqueous solution. It was
highly interesting to study the removal efficiency of a mixture of metal ions like Fe 3+ , Ni 2+ , Mn 2+ , Co 2+ , and Cu 2+ from lanthanides (La (III) and Nd (III) solution) using the regenerated GO-C adsorbent at pH = 2, as present in www.nature.com/scientificreports/  Comparison between the studied heavy metal ions onto other sorbents. The sorption capacity of the citrate-modified graphene oxide was compared with other sorbents reported in the literature ( Table 5). The data showed that the citrate-modified graphene oxide showed a significantly higher adsorption capacity for the studied metal ions. Therefore, it can be considered a highly effective material to adsorb these metals from an aqueous solution.

Conclusion
Citrate-modified graphene oxide (GO-C) was investigated to remove some heavy metals from lanthanides solution and characterized before and after adsorption using SEM, FTIR, Raman, and EDX. The modified GO-C showed rapid kinetics and an excellent adsorption capacity for Mn 2+ (223.22 mg/g), Fe 3+ (535.0 mg/g), and Ni 2+ (174.65 mg/g). The adsorption process using the modified (GO-C) is an endothermic and spontaneous reaction. Moreover, the GO-C can be reused over five times to enhance the efficiency of the removal process. In